Summary of Work: We use the powerful bacteriophage T4 model system to study basic mechanisms of mutagenesis and DNA repair. (1) When T4 replication is driven by the DNA polymerase-exonuclease of RB69, a distantly related phage, high fidelity is maintained. Thus, replication fidelity is likely to be determined primarily by the intrinsic fidelity of the polymerase-exonuclease and much less by polymerase interactions with the replication accessory proteins. (2) Forward mutation in T4 is often measured by counting r mutants. These arise in five genes. We have completed our role in a sequence analysis of the rI gene, are completing an analysis of the rV gene, and will establish a system for rapidly sequencing mutations in several of these genes. (3) T4 survival after DNA damage is higher at higher temperatures. Tests are underway to discover the distribution of this effect among T-even phages and its genetic determinants. (4) Certain alleles in two vital genes of T4 DNA metabolism define a DNA-repair epistasis group called replication repair. We are searching for mutations of this epistasis group in other genes of DNA metabolism. (5) In yeast, mutants of a putative RNase H are duplication mutators. In T4, an RNase H mutant is reported to be a mutator. We will determine the mutational specificity of such a mutator. (6) Mild heat and low pH promote mutagenesis in resting T4 particles. The mutational target is dG and the pathway is G:C -> T:A. We are testing the hypothesis that this process is also driven by trace metal ions.